Technical / research - Page 4

Researchers from Thailand's Mahidol University, Rajamangala University of Technology Thanyaburi and Synchrotron Light Research Institute have presented two novel air-stable hole transporting materials (HTMs) based on a spiro[fluorene-9,9′-xanthene] (SFX) core functionalized with N-methylcarbazole (XC2-M) and N-hexylcarbazole (XC2-H) rings. 

These HTMs were synthesized via a straightforward, three-step process with good overall yields (∼40%) and low production costs. To further reduce device cost, carbon back electrodes were employed. The resulting PSCs, with a structure of FTO/SnO₂/Cs_0.05FA_0.73MA_0.22Pb(I_0.77Br_0.23)₃/HTM/C, achieved power conversion efficiencies (PCEs) of 13.5% (XC2-M) and 10.2% (XC2-H), comparable to the reference spiro-OMeTAD device (12.2%). 

Researchers from Sweden's Chalmers University of Technology recently gained new insights into the dynamics of prototypical 2D halide perovskites (HPs) based on MAPbI₃ as a function of linker molecule and the number of perovskite layers using atomic-scale simulations.

The team showed that the layers closest to the linker undergo transitions that are distinct from those of the interior layers. These transitions can take place anywhere between a few tens of Kelvin degrees below and more than 100 K above the cubic–tetragonal transition of bulk MAPbI₃. 

Interface engineering plays a significant role in the constant improvement in the performance of perovskite photovoltaics, but such devices still suffer from several issues, including unavoidable open circuit voltage (VOC) losses. Now, an international team of researchers from Università Degli Studi Di Pavia, King Abdullah University of Science and Technology (KAUST), Chinese Academy of Sciences (CAS), University of Cambridge, Istituto Italiano di Tecnologia (IIT), Slovak Academy of Sciences and Imperial College London have proposed a different approach by creating a photo-ferroelectric perovskite interface. 

Graphical representation of the 2D/3D/2D perovskite heterostructure. Image from: Nature Communications

By engineering an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk, the scientists exploited the electric field generated by external polarization in the 2D layer to enhance charge separation and minimize interfacial recombination. As a result, they observed a net gain in the device VOC reaching 1.21 V, the highest value reported to date for highly efficient perovskite PVs, leading to a champion efficiency of 24%. 

Two-dimensional Ruddlesden-Popper (2DRP) phase perovskites have excellent long-term environmental and structure stability. However, the efficiency of 2DRP perovskite solar cells (PSCs) still lags behind that of their 3D counterparts due to the large exciton binding energy between the large-volume organic spacer and the inorganic plate compared to their 3D analogs.

To address this issue, researchers from China's Northwestern Polytechnical University and Xijing University have used a thin layer of self-assembled monolayer material between the transporting layer and the perovskite film for efficient and stable 2DRP-based PSCs. 

Slot-die coating (SDC) technology is a potential approach to mass produce large-area, high-performance perovskite solar cells (PSCs) at low cost. However, when the interface in contact with the perovskite ink has low wettability, the SDC cannot form a uniform pinhole-free perovskite film, which reduces the performance of the PSC.

Optimizing Slot-Die Coating for Commercial Solar Cell Production. Image credit: InfinityPV

Researchers from Korea's Jeonbuk National University have examined the correlation between interfacial roughness, wettability, and the overall efficiency of perovskite solar cells produced using slot-die-coating. This work offers a comprehensive understanding of how modifying the roughness of the hole transport layer (HTL) can improve the quality of perovskite films, enhance charge transport, and ultimately lead to high-efficiency perovskite solar cells with long-term stability.

Researchers from NingboTech University, Hunan Institute of Engineering, Hangna Nanofabrication Equipment Co. and University Malaysia Sabah have developed an inverted perovskite solar cell with an interface passivator based on lead carbanion (Pb–C^–), that reportedly achieved the highest open-circuit voltage ever recorded for an inverted perovskite PV device. The lead carbanion layer was responsible for reducing defects at the interface between the perovskite layer and the electron transport layer.

Inverted perovskite cells, or “p-i-n” cells, have the hole-selective contact p at the bottom of intrinsic perovskite layer i with electron transport layer n at the top. Conventional halide perovskite cells have the same structure but reversed – a “n-i-p” layout. In a n-i-p architecture, the solar cell is illuminated through the electron-transport layer (ETL) side; in the p-i-n structure, it is illuminated through the hole‐transport layer (HTL) surface. Inverted perovskite solar cells are known for their impressive stability but have been held back by relatively low efficiencies. This issue mainly arises at the point where the perovskite layer meets the electron transport layer, causing energy loss instead of being converted into useful power, primarily caused by carrier recombination, especially at the interface between perovskite and the electron transport layer.

Researchers from Chungnam National University and Russian Academy of Sciences have reported a method to increase the efficiency of perovskite solar cells (PSCs) by modifying the surface of a fluorine-doped indium tin oxide (FTO) substrate using an atmospheric pressure plasma treatment. 

The surface modification of the FTO film involved several challenges, such as control of the blocking layer uniformity, removal of pinholes, and deposition of a dense layer. This strategy allows the suppression of charge recombination at the interface between the FTO substrate and hole conductor. 

Researchers from Korea's Daegu Gyeongbuk Institute of Science and Technology (DGIST), Gyeongsang National University (GNU) and Kookmin University have developed a method to improve both the performance and the stability of solar cells using perovskite quantum dots. They developed longer-lasting solar cells by addressing the issue of distortions on the surface of quantum dots, which deteriorate the performance of solar cells.

A schematic diagram of bilateral ligand bonding on the surface of perovskite quantum dots. Image credit: Chemical Engineering Journal

Perovskite quantum dots can have excellent light-to-electricity conversion capabilities and are easy to mass-produce. However, according to the research team, in order to utilize them in solar cells, the ligands attached to the quantum dot surface must be replaced. This process often leads to distortions of the quantum dot surface, resembling crumpled paper, which results in decreased performance and shorter lifespans for the solar cells. To address this issue, the team adopted short ligands that securely hold the quantum dots from both sides, effectively uncrumpling the distorted surface. The ligands help restore the distorted lattice structure, smoothing the crumpled surface of the quantum dots. This significantly reduces surface defects, enabling the solar cells to operate more efficiently and extending their lifespan. Consequently, the power conversion efficiency of the solar cells increased from 13.6% to 15.3%, demonstrating stability by maintaining 83% of their performance for 15 days.

Researchers from CHOSE (Centre for Hybrid and Organic Solar Energy) at Tor Vergata University of Rome, CNR-ISM and Saule Technologies have introduced an optimized blade coating process for the scalable fabrication of large-area (15 cm × 15 cm) perovskite solar modules with a nickel oxide hole transport layer, performed in ambient air and utilizing a non-toxic solvent system. 

The research group fabricated a 110 cm² perovskite solar module with an inverted configuration and a hole transport layer that uses nickel oxide instead of commonly utilized poly(triarylamine) (PTAA). The proposed architecture aims to achieve high efficiency that is competitive with PTAA-based panels while improving stability.

Soochow University researchers have proposed the in situ treatment of Cl-rich benzene phosphorus oxydichloride (BPOD)as a way to achieve high-quality pure-blue perovskites, by simultaneously enlarging the perovskite bandgap, passivating the halide vacancy defects, and immobilizing the halide ions through the hydrolysis products of chloride ions and phenylphosphonic acid. 

The background for this work is that despite the substantial progress in sky-blue (480−495 nm) perovskite light-emitting diodes (PeLEDs), pure-blue PeLEDs (<480 nm) merely show moderate performances. Bromide-chloride mixed perovskites may have potential to enable a straightforward and effective way to obtain pure-blue emission, but the tricky issue of halide migration in mixed halide perovskites makes it challenging to achieve efficient PeLEDs with stable electroluminescence (EL) spectra.